Field of the Invention
The present invention relates to a radio sensing device and a radar system and, more particularly, to a radio sensing device and a radar system which perform wide-angle monitoring by using a plurality of short-range radar sensors which have been rapidly popularized in recent years to avoid the risks of vehicle-to-vehicle collision, vehicle-to-person collision, vehicle-to-railcar collision, and the like.
It is an object of the present invention to minimize the probability of radio interference in radio sensing devices which scan beam directions in a circular shape or a spherical-shell shape. For this reason, the invention cannot only be applied to a CTA (Cross Traffic Alert) radio sensing device but also be widely applied to a radio sensing device having the same scanning function as described above or a similar radio sensing device.
Description of Related Art
In recent years, an obstacle radar has been rapidly popularized. In addition to a well-known 60/77-GHz-band long-range radar (LRR: Long Range Radar) that linearly detects an area far from a vehicle front, a new 24/26-GHz short-range radar (SRR: Short Range Radar) that detects a rear area or a side area of a vehicle in a sector shape begins to be popularized.
A radio sensing device for CTA (Cross Traffic Alert) uses the SRR and is called one of typical techniques for vehicle. In 2013, the CTA has been already employed by five automobile manufacturers in Western markets, and the same function as that of the CTA has also been employed by domestic automobile manufacturers. About the CTA, see “Development of Automotive Active Safety System Using 24 GHz-band High Resolution Multi-Mode Radar”, Furukawa Review No. 132 (September, 2013).
An outline of the CTA will be described below with reference to FIG. 11. It is assumed that a plurality of vehicles (401 to 406) are parked without a parking space in a parking area and one of the vehicles is your own vehicle (403). In this state, when you try to drive your vehicle out of the parking area, your views in the horizontal directions are blocked by the vehicles (402 and 404) next to your vehicle to make blind spots. Since the blind spots are just on an aisle in the parking area, another vehicle (407) may dangerously come across the aisle in front of your vehicle. In particular, in a parking area in Europe or the United States, in contrast to in a parking in Japan, since a forward-parking and reverse-starting manner is popularly used, a drive of a vehicle in a reverse-starting state may easily pass over the vehicle (407) coming across the aisle in back of the vehicle.
Thus, in the conceived CTA, sector-shaped detection areas (411 and 412) are formed by radars installed at corner portions (in this case, rear corner portions) of a vehicle to try to detect the approaching vehicle (407) coming across the aisle.
One of the sector-shaped detection areas for CTA is popularly achieved by combining about four radio beams. An example of the structure of the combination is shown in FIG. 10. In this example, a radio sensing device including four antennas is built in each of the front and rear corner portions of your own vehicle (vehicle 1). Outputs of the four antennas are connected to a 4-to-1 switch circuit (321), and a switch control circuit (323) sequentially selects each of the four antennas as shown in a time chart in FIG. 10. In this case, it is assumed that a time required to sequentially select each of all the four antennas is defined as T. Since the four antennas are installed to have different directions in the form of a sector, radio beams are formed in different directions every moment to consequently cover sector-shaped detection areas (411 and 412) within the time T. That is, a sector-shaped detection area (411) is formed by the four beams, and a sector-shaped detection area (412) is also formed by four other beams.
A block diagram of a radio sensing device using a scheme which is similar to the above switch changeover scheme can be seen in, for example, FIG. 3, FIG. 4, and the like in M. Schneider, “Automotive Radar-Status and Trends”, pp. 144 to 147, GeMic 2005. Note that the 24-GHz-band SRR is used in not only the CTA but also a radio sensing device for other purposes such as an RVM (Rear Vehicle Monitoring) which is employed by domestic vehicle manufacturers. The 24-GHz-band SRRs of several types have been simultaneously mounted on one vehicle recently.
In contrast to this, in Japan, allocation of frequencies of 24-GHz-band radars is limited to a 200-MHz range from 24.05 GHz to 24.25 GHz as described in ARIB STD-T73. In this case, a bandwidth and a spatial resolution in radar sensing are in inverse proportion to each other, the 200-MHz bandwidth is hard to be divisionally used without sacrificing the spatial resolution. For this reason, under present circumstances, a plurality of radar systems coexist in the same frequency band.
In another arrangement that has been studied but has not been achieved, as shown in FIG. 13, independently of a radio sensing device mounted on a vehicle, for example, a system in which the same radar as that in the radio sensing device is installed at an intersection. In this system, a radio sensing device mounted on a vehicle performs proximity detection in back of the vehicle. The radio sensing device installed at an intersection performs proximity detection in front of a vehicle from the detection area of the radio sensing device to notify the corresponding vehicle of alert information. In this manner, proximity detection between a vehicle and a pedestrian, proximity detection between an advancing vehicle and a right- or left-turning vehicle, and the like can be performed.
However, a large number of vehicles in each of which 24-GHz-band SRRs of many types are mounted come into a city to disadvantageously cause radio interference between radio sensing devices of the vehicles. Naturally, the SRRs have been expected not to cause radio interference because the SRRs cannot distantly transmit radio waves. However, depending on a relative positional relationship between two vehicles and beam directions thereof, several situations in each of which short-time radio interference may occur can be conceived. Since a radio sensing device is a device directly linked to the safety, even under a rare condition, the risk of radio interference cannot be neglected. The probability of occurrence of the radio interference is required to be minimized. However, even for avoidance of radio interference, addition of new hardware undesirably increases costs. For this reason, a method of decreasing the probability of occurrence of wave interference without increasing the costs in the least has been desired.
In order to address the issue, “frequency hopping” which changes radio frequencies at random or a technique described in Japanese Unexamined Patent Publication No. 10-105228 were devised. Japanese Unexamined Patent Publication No. 10-105228 discloses a technique in which a radio wave having a wavelength equal to or larger than an extremely high frequency is transmitted to a reflecting tape on a roadway to detect a position of a vehicle with respect to a road surface. In this related technique, in order to prevent interference between two vehicles, a device that makes transmission timings of the vehicles different from each other to prevent the transmission timings from being matched with each other is conceived. However, since the related technique corresponds to so-called “time hopping” that consequently makes transmission timings on the time axis different from each other, as shown in FIG. 12, a cycle T′ becomes long. For this reason, a wasted time for which an antenna is not scanned is generated to cause the fear of deteriorating the safety.
An example in which the risk of occurrence of radio interference is present in the related technique will be described with reference to FIG. 10. In FIG. 10, in the vehicle 1, the four antennas are sequentially selected by the switch circuit (321) and the switch control circuit (323) to form a sector-shaped detection area. In a vehicle 2, four antennas are sequentially selected by a switch circuit (322) and a switch control circuit (324) to form a sector-shaped detection area.
It is known that, in a quasi-extremely-high frequency band such as a 24-GHz band, radio interference easily occurs when two antennas come close to each other or face each other. This situation, for example, as indicated by arrows in the drawing, can occur between two antennas in a region (335) when two vehicles are closely running in parallel with each other by chance. Although the two radio sensing devices of the two vehicles are not synchronized with each other at all, when control timings of the switch control circuit (324) and the switch control circuit (323) come close to each other by chance as shown in FIG. 10, radio interference may disadvantageously continuously occur in a plurality of time domains (331 to 334) in the drawing. The phenomenon described above may occur not only between vehicles, but also, for example, as shown in FIG. 13, between a radio sensing device installed at an intersection and a radio sensing device mounted on a vehicle.
Thus, the present invention has been made in consideration of the above issues, and has as an object to provide a radio sensing device and a radar system which reduce the probability of occurrence of radio interference with an adjacently arranged device the type of which is the same as that of the radio sensing device or the radar system or which is similar thereto without increasing costs and an occupied bandwidth.